Note: Descriptions are shown in the official language in which they were submitted.
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Specification 2 1 5 3 5 8 5
2
3 "METHOD AND APPARATUS FOR MASRING
4 THE ACOUSTIC SIGNATURE OF VESSELS"
6 HACRGROUND OF THE INVENTION
7 Field of the Invention
8 This invention relates to a method and apparatus for
9 masking the acoustic signature of surface and submarine
vessels, and more particularly, to the provision of a
11 resilient honeycomb cored cladding for the hull of a
12 vessel to provide acoustic masking.
13
14
16
17
18 Hackaround
19 One of the most common methods of detecting the
presence of surface and submarine vessels is the use of
21 sonar equipment. Sonar can be classified into two basic
22 types: i.e. active or passive sonar.
23 Equipment using active sonar detects the presence of
24 a vessel by emitting a short pulse of sound ("ping") and
monitoring the return of the pulse as it is reflected from
26 the hull of the vessel. Passive sonar equipment, on the
27 other hand, detects sound waves generated by the vessel as
28 it moves through the water, typically vibrations produced
29 by the vessel's propulsion system and other machinery on
board.
31 Both active and passive sonar systems are, if the
32 correct sophisticated equipment is used, able to produce
33 information relating to both the position and size of the
34 vessel detected. As such, sonar can be used to identify
the vessel and, as a result, the sonar characteristics of
36 the vessel are often called its acoustic signature. It
37 can, particularly in military applications, be important
38 to mask or alter the acoustic signature of a vessel, as
39 such masking will reduce the chances of detection and
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1 identification thereof.
2 One method of masking the acoustic signature of a
3 vessel that has been used in the past is known as the
4 Masker System. This system is in use by many NATO nations
and works by injecting air around the hull of the vessel.
6 Unfortunately, the Masker System only operates within a
7 narrow band of vessel speeds and is ineffective at speeds
8 above 15 knots or at very low speeds where the air bubbles
9 rise straight to the surface instead of streaming along
the hull. Certain sea conditions also adversely effect
11 the Masker System.
12 Furthermore, the Masker System increases the vessel's
13 wake and this can be disadvantageous if the vessel is
14 being attacked by wake-homing torpedoes. The Masker
System also requires a supply of compressed air which
16 itself can become a source of noise. Finally, air from
17 the Masker System can be sucked into the cooling water
18 intakes of the vessel, which can create additional
19 problems as the cooling systems become inefficient.
Yet another method of masking the acoustic signature
21 of a vessel is to install acoustic tiles in selected areas
22 on the hull of the vessel. These tiles come in two basic
23 varieties: (i) decoupling tiles which deaden the
24 sound produced by the vessel and
interrupt its transmission of sounds
26 into the water and thus help to defeat
27 identification by passive sonars; and
28 (ii) anechoic tiles which defeat active
29 sonar by reducing the amount of sonic
energy reflected from the hull by the
31 "ping" produced by this type of sonar .
32 Unfortunately, although effective in masking the
33 acoustic signature of a vessel, the acoustic tiles that
34 have been used to date are relatively thick (2 - 2.5
inches) and heavy (25 - 27 lbs. for a 2' x 2' tile). As
36 a result, the tiles are both expensive to produce and
37 expensive to install; the high installation costs being
38 due primarily to the additional labor costs and other
39 complications caused by the weight and thickness of the
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1 tiles. Furthermore, the tiles are difficult to secure in
2 place during installation and require the use of
3 complicated and expensive equipment.
4 There is thus a need for a lightweight and easy-to-
install acoustic tile which effectively and efficiently
6 masks the acoustic signature of a surface or submarine
7 vessel.
,
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1 SUMMARY OF THE INVENTION
2 Objects of this Invention
3 It is therefore an object of this invention to
4 provide a lightweight, flexible acoustic tile system for
installation onto the hull of a vessel to mask the
6 acoustic signature thereof.
7 It is a further object of this invention to provide
8 an acoustic tile which is made from a lightweight, durable
9 and resilient material.
It is yet another object of this invention to provide
11 an acoustic tile which is made of a flexible expanded
12 honeycomb-type material.
13 Still another object of the present invention is to
14 provide an acoustic tile of the type described which is
easy to install without requiring complicated expensive
16 installation equipment.
17 Yet a further objective of the present invention is
18 to provide a tile of the type described which can easily
19 be cut to size and is well suited to large-scale vacuum
shoring.
21
22 Summary of Invention
23 Briefly, according to a preferred embodiment of this
24 invention, an apparatus for masking the acoustic signature
of a surface or submarine vessel comprises a plurality of
26 tile-like components each including an inner skin for
27 securing the tile to the hull of the vessel, an outer
28 water-impervious skin, and a core sandwiched between the
29 outer and inner skins and made of an expanded honeycomb structure.
The acoustic tile of the invention also includes a
31 number of channels formed through in inner skin, each of
32 the channels being in communication with a vacuum port,
33 such that, when the tile is placed against the hull of the
34 vessel and a vacuum is applied to the vacuum port, the
resultant negative pressure in the channels serves to draw
36 the tile into intimate engagement with the hull of the
37 vessel. As a result, good contact and flow of the
38 adhesive used to affix the tiles to the vessel surface is
39 achieved.
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1 Preferably, the tiles are made of a thermo-plastic
2 urethane and are 0.625 inches thick and have plan
3 dimensions of 2' x 2'. Both the inner and outer skins of
4 the tile are fused to the honeycomb core, and the
thickness of the inner skin is slightly less than that of
6 the outer skin.
7
8 Advantages of the Invention
9 A primary advantage of this invention is that it
provides a lightweight tile which can be used to mask the
il acoustic signature of a surface or submarine vessel. The
12 light weight of the tiles means that no special lifts or
13 shoring equipment is required for installation.
14 Another advantage of this invention is that the use
of the acoustic tile reduces a vessel's reflectivity of
16 transmitted sonar energy back to receiving equipment and,
17 in turn, significantly reduces the range at which active
18 detection of the vessel can be made.
19 Yet another advantage of this invention is that, in
addition to reducing a vessel's acoustic signature on
21 hostile passive sonars, use of the present invention adds
22 to the sensitivity of the vessel's own sonar by causing
23 the vessel to generate less self-noise into the sea.
24 Thus, operators of on-board sonar have less background
noise to work against.
26 Still a further advantage of this invention is that
27 the acoustic tiles of the present invention require no
28 additional equipment or piping, as is required by the
29 Masker System, to provide the acoustic masking function
required.
31 An additional advantage of the present invention is
32 that it tends to reduce hull surface drag due to the
33 undulating dynamics of the tile.
34 These and other objects and advantages of the present
invention will no doubt become apparent to those skilled
36 in the art after having read the following detailed
37 description of the preferred embodiment illustrated in the
38 several figures of the drawing.
39
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1 DESCRIPTION OF THE DRAWINGS
2 In the accompanying drawings:
3 Fig. 1 is a broken perspective view illustrating in
4 part an exemplary hull of a vessel having acoustic tiles
in accordance with the present invention mounted thereon;
6 Fig. 2 is a partially broken perspective view
7 illustrating an acoustic tile in accordance with the
8 present invention;
9 Fig. 3 is a plan view showing details of the inner
skin of an integrated fairing tile in accordance with the
il present invention; and
12 Figs. 4-8 are enlarged cross-sections taken along the
13 line 4-4 in Figure 3.
14
BRIEF DESCRIPTION OF A PREFERRED EMBODIMENTS
16 Referring now to Fig. 1 of the drawing, an acoustic
17 coating system in accordance with the present invention
18 applied to a ship hull 10 is shown generally at 12. The
19 system is comprised of a plurality of rectangular
(preferably square) tiles 14 affixed to the surface of the
21 hull 10 by a suitable adhesive, and further includes a
22 plurality of tapered fairing tiles 16 distributed around
23 the system perimeter to provide a smooth transition from
24 the hull surface to the outer surface of the tiles 14.
As is well known in the art, such tiles 14, 16 would
26 normally be strategically positioned to cover selected
27 areas of the ship's hull dependent upon whether the
28 coating is intended to provide a decoupling function or an
29 anechoic function, or both.
The general structure of the acoustic tile of this
31 invention is illustrated at 20 in Fig. 2 of the
32 accompanying drawing. Typically, the tile is a square of
33 approximately 2' x 2' in plan dimension, and is comprised
34 of a relatively thin inner skin (or "under-ply") 22, a
honeycomb core 24 and a relatively thick outer skin (or
36 "cover-ply") 26. The skin 22 is typically a sheet of
37 0.040" (or up to 0.100") thick thermoplastic elastomer
38 (TPE), the core 24 is typically an expanded 0.500" thick
39 honeycomb of TPE having wall thicknesses of 0.012" and
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1 cell diameters of 0.375", and the outer skin 26 is
2 preferably a sheet of 0.125" thick TPE. The tiles are
3 constructed using the method described in our earlier U.S.
4 Patent No. 5,039,567 entitled "Resilient Panel Having
Anisotrophic Flexing Characteristics and Method of Making
6 Same")
7
8 To facilitate attachment of the tiles to the ship
9 hull, circumscribing grooves or channels 28 and crossing
grooves 30 (0.125" wide) are formed in or through the
11 inner skin 22 and into the core 24 to a depth of
12 approximately 0.125". Layers of the tile are broken away
13 in the foreground, and dashed lines 32 are used to
14 illustrate the depth of the channels.
Similarly, a mid-portion of the skin 22 and
16 underlying core 24 are also broken away to reveal a small
17 vacuum port 34 passing through the outer skin 26 to
18 communicate with the grooves 30 and 28.
19 Additional channels can be cut on site to accommodate
odd sections of the hull, and can be arrayed in any
21 suitable pattern, e.g., in straight lines, diagonal lines,
22 zig-zag patterns, etc. Furthermore, the cells opened by
23 the channel forming cuts may be filled with a suitable
24 filler material so as to avoid collapse of those cells
during vacuum shoring.
26 Turning now to Figs. 3 through 8, the details of a
27 corner fairing tile 16 will be described. Note that Fig.
28 3 is a plan view of the inner skin side and Figs. 4
29 through 8 are broken cross-sections taken along the line
4-4 in Fig. 3. . The fairing tile is comprised of a
31 generally flat area 40 and a gradually tapering portion 42
32 disposed along two sides of the tile. A simple edge
33 fairing tile would obviously taper only along one side.
34 The tapered portion 42 serves to streamline the tile so
that it provides a minimum flow resistance to the vessel
36 hull as it moves through the water.
37 Like the previously described rectangular tile, the
38 typical fairing tile includes a thin inner skin 44 and a
39 thicker outer skin 46 (Fig. 4) which sandwich between them
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1 a core 48. The thin inner skin 44 is typically 0.04" to
2 0.100" thick, while the thickness of the outer skin 46 is
3 typically at least 0.125" thick. The core at the flat
4 portion 40 of the tile is approximately 0.5" thick and
comprises a honeycomb structure of the type described
6 above. The tapered core portion 42 is identical in
7 configuration to that of the flat portion (and may in fact
8 be an integral part of portion 40) except that it has been
9 shaved, trimmed, or otherwise altered to provide the
desired taper. The inner and outer skins 44 and 46, and
11 the core 48 are all made of a thermo-plastic urethane and
12 are fused together to form a single, watertight, unitary
13 construction, i.e., an integrated acoustic tile and
14 fairing piece.
Both the flat and tapered portions 40, 42 of the tile
16 have a number of grooves or channels 50 formed in or
17 through the inner skin 44. As is apparent from the
18 drawing, these channels are all interconnected and lead
19 either directly or indirectly to a vacuum port 52 formed
through the honeycomb and outer skin of the tile. The
21 port 52 can be of any suitable configuration ranging from
22 a simple puncturable membrane to an elaborate valuing
23 structure. In some cases it may be desirable to use a
24 somewhat thicker inner skin 53 as depicted in Fig. 5, so
that the channels 50 can be molded or cut thereinto. As
26 one example, the inner skin, or underply, 53 was made of
27 a layer of material 0.090" thick with 0.070" deep channels
28 50. The benefit of using this format for the tiles and
29 fairing tiles is that when the shoring vacuum is applied,
the tiles will npt experience dimpling.
31 The tapered portion 42 of the tile can be constructed
32 in several ways: One method is to pre-form or trim the
33 honeycomb core 48, as described above, and thereafter cast
34 a pourable thermo-plastic urethane to form the sloping
outer skin. As an alternative, the entire sloping portion
36 42 can be made of a cast elastomeric material or foam
37 which, as shown at 43 in Fig. 6, is then joined to the
38 rectangular core 40 at line 54, the transition between the
39 sloping portion 42 and the flat portion 40. The cast
~~53~'$5
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1 elastomer would have similar acoustic masking properties
2 to the honeycomb core making up the rest of the tile.
3 However, as the sloping portions 42 represent a relatively
4 small percentage of the total area on the hull covered by
acoustic tiles, these properties need not be as good as
6 that of the honeycomb structure. In addition, the
7 fairings are usually angled and would, in any event, not
8 reflect sound from active sonar as well as other parts of
9 the hull of the vessel.
As depicted in Fig. 7, it is sometimes preferable to
11 fill the cells 56, that have been opened during the
12 channel forming process, with a suitable material, so as
13 to prevent partial collapse of those cells during the
14 vacuum shoring process. In addition, it may also be
desirable to bond an inner liner 58, having preformed
16 channels 60, to the skin 44. Alternatively, a plastic
17 elastomer could be cast over the scored inner side to
18 accomplish a result similar to that achieved by the layer
19 58.
In order to save costs, it may be desirable to use
21 yet another configuration such as is depicted in Fig. 8,
22 wherein the outer skin 46 is shaped, in the fairing tile
23 case, and smaller honeycomb pads 70 and 72 are positioned
24 thereover and then cast in place with a suitable resilient
material 74. Note that in order to accommodate the
26 tapered edges, thinner pads 72 with the same, or a
27 different, cell size may be stacked as indicated.
28 When the tile is to be secured to the hull of a
29 vessel, the inner skin 44 is first abraded or treated with
a urethane primer. An adhesive, preferably an epoxy or a
31 pressure-sensitive adhesive, is then applied thereto (and
32 to the hull) and the tile is positioned and held in place
33 against the hull. A vacuum is then applied at the port 52
34 to reduce the pressure in the channels 50 and cause the
tile to be drawn into firm engagement with the hull. The
36 vacuum port 52 can then be sealed off and the tile held in
37 position by the combination of adhesive and vacuum or,
38 alternatively, if the vacuum is removed, by the adhesive
39 alone. As the tiles are installed in side-by-side
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1 relationship, a suitable adhesive or silicone caulk is
2 applied at the tile edges to permanently seal the
3 junctions between the tiles. It is important to note that
4 the entire panel must be air-tight so that in use no sea
water is able to leak into or under any part of the panel.
6 Once the tiles have been fitted onto the hull of the
7 vessel, the outer skin 46 can be coated with an anti-
8 foulant, which reduces the amount of marine growth on the
9 exterior on the hull of the vessel.
Although the present invention has been described
11 above in terms of a specific embodiment, it is anticipated
12 that alterations and modifications thereof will become
13 apparent to those skilled in the art. It is therefore
14 intended that the following claims be interpreted as
covering all such alterations and modifications as may
16 fall within the true spirit and scope of the invention.
17 What is claimed is:
